Steel is refined in subsurface pneumatic refining vessels of many different sizes ranging from very large vessels capable of refining a heat of steel weighing 300 tons, to small vessels capable of refining a heat of steel weighing about five tons. Lately there has arisen a need to refine very small heats of steel weighing about two tons or less. Consequently there is a need for steel refining vessels sized to accommodate such very small heats.
At first glance it might appear that such a problem is easily solved by simply building a proportionally smaller steel refining vessel of the known design. Such a procedure has heretofore been effective in producing steel refining vessels of various sizes. For example, a 150 ton steel refining vessel and a 5 ton steel refining vessel have about the same design parameters despite their size difference.
A major problem in subsurface pneumatic steel refining is retaining enough heat within the steel melt during refining to ensure that the refined steel melt will be at the proper tap temperature after refining. This is because heat from external sources generally is not added to the melt during refining. Although some heat is generated by exothermic refining reactions such as decarburization or the oxidation of fuel elements, the melt during refining can experience a net heat loss. If the heat loss is such as to cause the melt to be below the proper tap temperature, the melt must undergo a time consuming and expensive reblow in order to attain the proper tap temperature.
Herein lies a major problem in the design of a very small steel refining vessel. As is well known, the heat loss of a mass is directly related to the ratio of its surface area to volume, i.e., the greater is the surface area of the mass for any given volume, the greater will be the rate of temperature loss of the mass. As steelmaking vessels of known design are made proportionately smaller, their surface area to volume ratio increases and thus the rate of temperature loss increases. This problem is even more acute when the AOD, or argon-oxygen decarburization, process is employed because of the use of inert diluent gas during refining which further contributes to heat loss. The AOD process is a preferred steel refining process due to the cleanliness and pinpoint constituent accuracy of steel refined by this process.
Another major problem in the design of a very small steel refining vessel is the need to achieve a conducive gas liquid interface and gas residence time for efficient gas metal reactions. Especially when employing the AOD process it is advantageous to maintain a sufficient volume of molten metal above the point at which the refining gases are injected into the molten metal in order to obtain efficient utilization of injected gases used for removing impurities by degassing, deoxidation, volatilization or by flotation of said impurities with subsequent entrapment or reaction with the slag and gases used for alloying.
Examples of known subsurface pneumatic steel refining vessels can be found in many references including U.S. Pat. No. 3,724,830--Molten Metal Reactor Vessel, U.S. Pat. No. 3,816,720--Process For The Decarburization of Molten Metal and U.S. Pat. No. 4,208,206--Method For Producing Improved Metal Castings By Pneumatically Refining The Melt.
Accordingly, it is an object of this invention to provide an improved subsurface pneumatic steel refining vessel which will enable one to more efficiently refine a heat of steel weighing about two tons or less.
It is a further object of this invention to provide an improved subsurface pneumatic steel refining vessel which will enable one to more efficiently refine a heat of steel weighing about two tons or less by use of the AOD process.
It is another object of this invention to provide an improved subsurface refining method to efficiently refine a heat of steel weighing about two tons or less.